I'm not sure what would yield the best results, but if it were me, it would be crits, master's swimming, 5ks, cx races, trail runs, swim meets, mtb races etc. Become skilled in all three disciplines and build a fitness base while having fun doing it.

Three years of prescribed training trying to hit power intervals would probably make me hate the sport.

Xterra races. Once you go dirt, you will never go back! :)

Honestly, enjoy the kids and do enough to keep sane / reasonably fit. I would not cut the running completely due to the leg changes with/without the impact. Also, running can take much less time.

Get as fit as possible with the hours you got. I would do mostly running since you said it's your weakest sport, if the idea is to do a good IM in a few years. But focus on what is fun and love the journey.

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Endurance coach | Physiotherapist (primary care) | Bikefitter | Swede

Race Olympic distance for 1-2 years and maybe throw in a "fun" 70.3 in 2-3 years just to ease back into the IM ramp, very achievable with 8 hrs/week given your athletic potential if you were crushing out a 4:30 70.3 a few years ago. You could probably do 70.3 on 8 hrs with just a handful of 10 hr weeks. Maybe sprinkle in a few trail 1/2 marathons to build lower-joint stress running strength since it's your limiter.

I don't think you should be a single sport athlete. You want to preserve swimming feel and maintain strength on bike and preserve triathlon racing mindset/skills (fast transitions, etc.)

Get a coach who can help you use time efficiently and get the most out of your sessions. I've been very happy with Purple Patch, my coach does an amazing job building my plan around my life and available hours.

Make that time count.
Don't mess around with junk miles if you only have 8 hours a week.

If you can find time, get out for some longer rides and runs. I know a lot of dads who get up really early and knock out a couple hours.

And try to find the time to do some short races. Racing even 5k's and stuff keeps you sharp.

If you are a pretty good swimmer and biker, focus on the run. I'd also focus on doing some open runs and getting faster at the Olympic and 70.3 distance. I did this for three years between my last two IM races.

I made a point after my IM in 2015 to not do another IM until I felt I'd be able to have a good all around race. I took almost ten minutes off my best 70.3 time over that span. I raced IMLP this summer, but unfortunately picked up a running injury two weeks before the race. I'd hoped that somehow things would be ok on race day. I was the 5th amateur onto the run, but had to withdraw just over halfway through the run as the injury forced me to slow drastically, and I didn't want to walk 12+ miles.

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I agree wholeheartedly. Get fast but stay short in all three disciplines which can be accomplished in the 8 hours per week that you can afford. Even make it seasonal doing swim focus in the winter with swim meets, CX races in early spring and some running, Crit's and TT's in the summer and then 5K's in the fall.

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USA Triathlon Level 2 Coach
Slowtwitch Master Coach
Head Coach, TriCoach Colorado, LLC

Get a Burley Wagon or similar piece of equipment usable for bike and running. Take your kids on your workouts as often as you can. You'll be building bonds with the kids and strength in your body. Take the kids to the pool for dad & kid classes or even lessons when old enough. Build up some parenting points with these activities, otherwise no deposit and no return.

Join masters for a weekly swim to maintain skills and feel of the water. Put more miles in on the run than now to build up running capabilities. You'll appreciate your time investment when you start training during the IM block. Otherwise, you may be biased to go longer on the bike, since that's your background strength, and think to let the run be what the run will be. When you talk with others about what to expect during the IM race and about IM race tactics, you will hear from many that the race doesn't really get serious until the 80 mile marker on the bike. When you start experiencing racing beyond that point, you will appreciate all the miles of running you put on you legs between now and then. Vary the time, distance, bike to run ratios, and whatnot.

Keep all this fun for you and family. Everyone will be happier. Race if you want. Go Sprint and Olympic distances. You can put in some good races at 8 hours of training in a week. Though you mentioned you anticipate little or no racing. You won't miss much in IM prep for racing. You know what to experience during races based on your 4:30 HIM and you probably did a lot of cycling races. You only really need to do what you want to feed the competitive beast within during the time between now and your IM debut.

In 2-3 years start cashing in your points that you banked. You need them to keep the family support during a 12-15 week IM build-taper-peak block period. You'll want to double your hours (16-20) of training at about mid-point to five weeks out of race during the block. On race day, cash the reminder in.

Pros and elite take breaks for multiple seasons without serious training periods and return to reason at the top of the podium. Really, eight hours is great to maintain a base before entering into a go for serious IM training. There's a lot of triathletes who would like that many hours to train for much shorter distances.

https://www.palmtreesahead.com/
https://www.palmtreesahead.com/tactical-learning

I would go full Endurance Nation style.
Swim: Deprioritize swim since you said you were a strong swimmer (don't swim in winter, only swim to prep for races). Masters x2 per week will get you really far.

Bike: Get TrainerRoad and do Sweet Spot Base to get started then just repeat Sustain Power Build as many times as you can (e.g. get your 1 hour power as high as possible). 2x 1 hour during week and 1 weekend ride of 90 mins will be plenty if you focus on Z3 to Z5.

Run: Go full BarryP. Start with a volume that works for you and build from there. Build to 50 miles per week. I'd throw in some strides x2 per week and 1 day a week of HalfMary paced intervals (nothing too hard).

Weekly schedule (assuming no swim) would look like:
Mon - Off or Yoga or strength training
Tues - 1 hour hard bike, 30 min run
Wed - 60 min run
Thu - 1 hour hard bike, 30 min run
Fri - 60 min run
Sat - 90 min hard ride
Sun - 90 min run

That gives you 8 hours of solid training per week and very doable with family (say hello to early mornings before work and late nights after everyone is in bed)

2-3 years out? Don't over-think it, just train for fun for the first year, but be consistent.